A flexible metal laminate is mainly used as a substrate of a flexible printed circuit board, and it is also used as flat heating element electromagnetic shield material, flat cable, package material, and the like. Among the flexible metal laminates, a flexible copper clad laminate consists of a polyimide layer and a copper foil layer, and it is divided into an adhesive type and a non-adhesive type according to the existence of an epoxy adhesive layer between the polyimide layer and the copper foil layer. The non-adhesive flexible copper foil laminate is formed by direct adhesion of polyimide to the surface of the copper foil, and with the recent tendency of miniaturization and slimming of electronics, and requirement for excellent ion migration, the non-adhesive flexible copper clad laminate is predominantly used.
And, with the tendency of miniaturization, high speed and combination of various functions of electronics, there has been a demand for improving signal transduction speed of electronic device. Thus, there is a demand for development of a printed circuit board using insulator having lower dielectric constant and dielectric loss than the existing insulator. Recently, reflecting the tendency, there have been attempts to use liquid crystalline polymer (LCP), which is an insulator having lower dielectric constant than the existing polyimide and is influenced little by moisture absorption, in a flexible printed circuit board. However, dielectric constant of LCP (Dk=2.9) is less excellent than dielectric constant of polyimide (Dk=3.2), it has too low heat resistance, and it has low compatibility with a PCB manufacturing process using the existing polyimide. Therefore, studies on lowering dielectric constant of polyimide have been progressed.
For example, U.S. Pat. No. 4,816,516 describes technology of manufacturing molded product by mixing polyimide and fluorine-containing polymer. However, it has limitation that it directly uses polyimide having high coefficient of thermal expansion and low Tg (glass transition temperature). And, to use for a printed circuit board, it is required to be manufactured in the form of a thin film, but the above patent does not mention a metal laminate in the form of a thin film. And, U.S. Pat. No. 7,026,032 describes a method of lowering dielectric constant by dispersing fluoropolymer fine powder in polyimide. However, it has problems in that adhesion to a coverlay or adhesion to prepreg decreases. And, since CTE values in the Examples of the above patent are too large, it has a limitation for use as a flexible metal laminate. Moreover, since fluoropolymer is exposed on the surface, the fluoropolymer is dissolved during a high temperature storage process, thus causing a risk of delamination of copper foil circuit from an insulator.